The invention relates to an echo cancelling arrangement for processing a signal occurring in a first one-way path for cancelling a composite echo signal occurring in a second one-way path in response to said signal in the first path, said composite echo signal being formed by several components having different characteristics.
An echo canceller is used in, for example, a transceiver arrangement, whose one-way transmit and receive paths, which together form a four-wire circuit, are often coupled by a circuit known as a hybrid junction in a manner such that the arrangement has a two-wire access to the exterior. It is known that when a connection is established between two transceiver arrangements by their two-wire accesses, an echo signal, created by the signal in the transmit path of an arrangement and due to imperfections of the hybrid junctions and/or signal reflections in the connection, may be untimely produced in the receive path of the same arrangement. An echo canceller has for its object to automatically cancel this untimely echo signal appearing in the receive path. The advantage of this echo cancelling operation is that it enables a simultaneous transmission in both directions between two transceiver arrangements intercoupled by the two-wire accesses. The transceiver arrangements are, for example, modems used for data transmission.
Depending on the nature of the echo path, that is to say the path through which the signal from the transmit path passes in order to produce an echo signal in the receive path, this echo signal may have different characteristics.
When in this echo path only operations of a linear character occur which do not cause the phase or the frequency of the signal to change, a what is commonly referred to as a linear echo is involved which can be cancelled by an echo canceller which, by means of a linear operation performed on the signal in the transmit path, can automatically produce a simulated echo signal which is subtracted from the signal in the receive path. This type of linear echo canceller is generally known and described in detail in the literature: see, for example, French Patent Specification No. 2,377,734.
There are however echo paths producing echo signals having other characteristics. If, for example, a carrier system is included in the echo path, it may happen, in certain situations, that this echo signal has a frequency different from the frequency of the transmitted signal and consequently a time-variable phase. It is likewise possible that the echo path performs non-linear operations; this is, for example, the case when the echo path comprises compression-expansion systems which do not perfectly cancel each other or amplifiers producing harmonic distortion. It is not possible to solve the problem of cancelling an echo having such characteristics in a satisfactory manner by means of the conventional echo cancellers. See for this subject, for example, the article by E. J. Thomas, entitled "An adaptive Echo Canceller in a Nonideal Environment (Non-linear or Time Variant)" published in Bell System Technical Journal, Vol. 50, No. 8, October 1971, pages 2779-2795. Therefore, echo cancellers have recently been introduced which are especially arranged for cancelling an echo signal having a given characteristic. For instance, United States Patent Specification No. 4,072,830 or U.S. application Ser. No. 704,895, filed by Applicant on Feb. 22, 1985, which was a continuation of U.S. application Ser. No. 581,223, filed by Applicant on Feb. 22, 1984, which was a continuation of U.S. patent application Ser. No. 247,195, filed by applicants on March 24, 1981, which claimed priority of French patent application No. 80 06 748 filed March 26, 1980, which U.S. application is incorporated herein by reference and is referred to herein as 06/704,895 describe an echo canceller for a variable-phase echo signal. However, in practice, an echo signal may often by of a composite nature, that is to say it may be formed by the sum of several echo signals having different characteristics. So it rarely happens that a variable-phase echo signal is not accompanied by an echo signal having a linear characteristic. To all appearances, so far the specific problem of cancelling a composite echo signal has not yet been looked into.